Electrical energy storage cells are widely used to provide power to electronic, electrical, electromechanical, electrochemical, and other useful devices. Such cells include primary chemical cells, secondary (rechargeable) chemical cells, fuel cells, and various species of capacitors. Important characteristics of electrical energy storage cells include energy density, power density, charging rate, internal leakage current, equivalent series resistance (ESR), and ability to withstand multiple charge-discharge cycles. For a number of reasons, double layer capacitors, also known as supercapacitors and ultracapacitors, are finding use in applications that traditionally have been filled by batteries. These reasons include availability of double layer capacitors with high power and energy density characteristics that respectively exceed and approach those of conventional battery technology. In other words, double layer capacitors are capable of delivering high instantaneous power levels, can be quickly recharged, and store substantially more energy per unit weight and unit volume than battery cells.
Different kinds of electrical energy storage devices generally have different profiles of voltage-versus-charge curves. Some chemical cells, for example, maintain a relatively flat (constant) voltage throughout their useful life, and then suffer a steep voltage drop with continuing discharge. In the case of some battery cells, discharge below a certain voltage level may cause permanent damage to the cells, with these types of cells generally being discharged below a predetermined voltage level during operation. In contrast, capacitor voltage decreases linearly with the charge drawn from the capacitor, according to the following well-known formula:
            ⅆ      V              ⅆ      t        =            I      C        =                                        ⅆ            q                                ⅆ            t                          C            .      
In the above formula,
      ⅆ    V        ⅆ    t  is the time derivative of the capacitor voltage, I is the current flowing into the capacitor (so that a discharging capacitor corresponds to a negative I), C is the capacitance value of the capacitor, and
      ⅆ    q        ⅆ    t  is the time derivative of the charge stored in the capacitor.
Electrical devices are limited in their operation by the specified operating voltage range of their respective power sources. (Electrical devices or loads in the present context are defined broadly to include any device that uses electrical energy in the course of its operation, including, without limitation, electronic, electromechanical, electrochemical, and other devices.) As discussed, when discharged below some minimum voltage, battery cells may be damaged, which acts to limit the operating voltage range over which an electrical load device connected thereto can be operated. For example, in an automotive starter and battery application it is known that full discharge can destructively affect the starter battery. For this reason, automotive starter batteries are maintained in a state that is above a certain minimum voltage. Although operation below such a minimum voltage is can be avoided, it entails that the full range of energy available from the battery is cannot be utilized.
Double layer capacitors, on the other hand, can be repeatably discharged to zero volts without experiencing any damage. For this reason, use of double layer capacitors can enable the use of electrical load devices over a wider operating voltage range.
However, matching a double layer capacitor cell voltage profile to electrical load device requirements presents additional considerations. As has been mentioned, capacitor voltage decreases linearly with the charge drawn from the capacitor. Thus, even if an operating voltage range is broad enough so that the upper voltage limit is twice the lower voltage limit
      (                  V        upper            =                        2          *                      V            lower                    ⁢                                          ⁢          or          ⁢                                                            ⁢                                                          ⁢                      V            lower                          =                              V            upper                    2                      )    ,twenty-five percent of the cell's energy will remain unused when the capacitor voltage drops to the level of the lower voltage limit. This statement can be verified from the formula relating the energy stored in the capacitor (E) to the capacitor's voltage (V) and capacitance value (C):
  E  =                    C        *                  V          2                    2        .  
Thus, capacitor energy remaining at the lower voltage limit (Elower) can be expressed in terms of the capacitor energy at the upper voltage limit (Eupper) as follows:
      E    lower    =                    C        *                  V          lower          2                    2        =                            C          *                                    (                                                V                  upper                                2                            )                        2                          2            =                                    C            *                          (                                                V                  upper                  2                                4                            )                                2                =                              0.25            *                                          C                *                                  V                  upper                  2                                            2                                =                      0.25            *                                          E                upper                            .                                          
This energy value may be a significant, and generally constitutes a higher percentage of remaining energy than at a comparative battery voltage.
In many applications the energy remaining in a battery or capacitor cell presents a safety concern. For one example, the cell may need to be shipped for disposal, to a recharging facility, or elsewhere; or the cell or electrical load device connected thereto may need to be serviced. The cell may also be desired to be used and/or stored in an inherently dangerous environment, such as an oil platform, a mine, an explosives factory, or a fireworks factory. In these and other similar environments, the potential for arcing or sparking may present a high degree of risk to life and property. Indeed, the risk may be unacceptable. For example, in the oil platform environment, there is a requirement that there be no sources of arcs or sparks that could cause ignition of combustible gases and material. During certain scenarios, for example as during oil platform maintenance, this requirement dictates that all sources of energy be discharged to zero. The requirement for full discharge prevents the use of battery technology on oil platforms, which in turn, prevents oil platforms from using batteries as backup power sources, for example, as for power backup of computer systems during power failure. Consequently, when there is a power failure, one or more systems on an oil platform can be completely disabled.
Because of their ability to provide performance similar or better than battery cells, double layer capacitor cells have been used in the prior art for backup power. Unlike battery cells, double layer capacitor cells can be discharged to zero volts. However, the rate of such discharge is limited by a capacitors RC time constant as well as by the resistance of the discharge circuit. For example, when shorted using a 10 foot 4 gauge battery cable with an internal resistance of 4 milliohm, a 48 volt 400 Farad capacitor source with a 4 milliohm internal DC resistance comprises a circuit with an RC time constant of about 3.2 seconds. In practice, to limit discharge current to a lower level, a low resistance power resistor is used instead of battery cables. With increased resistance, there occurs increased discharge time to a “safe” level, which when attempted using a passive circuit comprising a passive 1 ohm high power value resistor can be on the order of many hours, if not days. Thus, because a “nearly depleted” double layer capacitor cell does not mean a “completely depleted” cell, to date, double layer capacitors have been limited to use in applications where long discharge times have not been a major safety issue.
Therefore, it would be desirable to provide one or more solution to the problems presented in the prior art.